1. Field of the Invention
This invention relates to wet seals generally, and in particular, wet seals for use in high temperature fuel cells. Fuel cells have been widely sought for their ability to directly convert chemical energy into electrical energy through electrochemical means. In molten carbonate fuel cells, the chemical energy of hydrocarbons is converted directly into electrical energy by a, galvanic oxidation/reduction process.
Generally, fuel cells are comprised of a multiplicity of individual cells, separated by inert or bi-polar ferrous metal separator plates. The individual cells are sandwiched together and secured into a single unit to achieve the desired fuel cell energy output. Each individual cell generally consists of an anode and cathode electrode, a common electrolyte tile and a fuel and oxidant gas source. Both gas sources are introduced through manifolds to their respective reactant chambers formed by peripheral spacer strips between the separator plate and electrolyte tile. It is the area of contact between the electrolyte and the separator plate peripheral spacer strips or the fuel cell housing which is known as the wet seal.
A major factor attributing to premature fuel cell failure is corrosion and fatigue of the fuel cell housing, especially in the wet seal area. This mode of failure is hastened by corrosive electrolyte contact at high temperatures and high thermal stresses placed on the fuel cell members, especially thermal stresses resulting from large temperature variations of thermal cycling, thereby causing weakening of the fuel cell structure through intracrystalline and transcrystalline cracking. These failures permit gas crossover. Fuel and/or oxidant gas crossover interrupts the intended oxidation and reduction reactions thereby causing breakdown and eventual stoppage of cell current generation.
Molten carbonate electrolytes are very corrosive to ferrous metals which are required for their strength in fuel cell housings and separator plates. The high temperature operation of molten carbonate fuel cells increases both the corrosion and thermal stress problems in the wet seal areas, especially when the thermal coefficients of expansion of adjacent materials is different.
2. Description of the Prior Art
In recent years considerable effort has been made to provide effective sealing to prevent premature failure of fuel cells due to corrosion of the fuel cell housing in the wet seal regions in contact with the fuel cell electrolyte and to provide physical stability with high temperature thermal cycling.
One method utilized to mitigate the corrosion rate of the fuel cell housing referred to in this disclosure and claims as "separator plate" to refer both to the external cell housing of end cells and separators of internal cells of a stacked assembly, is to bond, by welding or brazing, solid metal corrosion resistant strips to the ferrous metal separator plate in regions susceptible to electrolyte corrosion. These metal strips typically are an expensive, high temperature metal alloy suitable to withstand the corrosive environment of the fuel cell electrolyte. These metal alloy strips serve as inert wet seals to prevent direct contact of the electrolyte with the ferrous separator plate preventing galvanic corrosion of the separator plate. To provide intake and exhaust manifolds, the metal alloy strips must be machined or drilled.
A number of disadvantages have resulted where metal alloy strips are welded or brazed to the separator plate to provide a non-corrosive wet seal. Often, distortion and warpage to the separator plate itself results from the localized high thermal gradients and stresses experienced during the process of joining the high temperature alloy strips to a thin separator plate. A warped or distorted separator plate in turn, loses much of its intended ability to maintain complete oxidant and reactant gas chamber separation and leads to premature failure of the fuel cell. Because the separator plate also serves as the primary structural load bearing member of the fuel cell, distortion of this structural member may cause the entire cell assembly to shift and distort resulting in the likelihood of cracking of the electrodes and/or electrolyte tile.
A wide variety of attempts have been made to prevent electrolyte corrosion of a fuel cell housing or separator plates to provide stable long term cell operation. These problems are most severe with corrosive electrolytes and high temperature thermal cycling.
U.S. Pat. No. 3,723,186 discloses stacked high temperature electrochemical cells wherein the electrolyte itself is comprised of inert materials in regions about its periphery, to establish an integral inert peripheral seal between the electrolyte and fuel cell housing. This type of wet seal requires strict tolerances be maintained in the manufacture of an electrolyte containing these limited regions of inert material. Should these tolerances slightly vary, undesired electrolyte creepage or gas crossover may occur. Moreover, should the electrolyte contain inert material in excess of tolerance limits, blockage of the intended oxidation/reduction reactions occur in regions where the electrolyte, gas and electrode meet. Where this happens, the energy output of the cell may be significantly reduced.
U.S. Pat. No. 4,160,067 discloses another wet seal for high temperature molten carbonate fuel cells wherein inert materials are deposited directly onto, or impregnated into, the fuel cell housing. U.S Pat. No. 4,329,403 teaches an electrolyte-electrode assembly for high temperature fuel cells in which the molten carbonate electrolyte has a graded composition for more gradual transition in the coefficient of thermal expansion in going from the electrodes to the inner electrolyte region. U.S. Pat. No. 3,514,333 teaches housing of molten carbonate electrolytes in high temperature fuel cells by use of a thin aluminum sealing gasket.
The sealing and corrosion problems are not as severe in the wet seal areas of low temperature electrolytic cells. U.S. Pat. No. 3,867,206 teaches gas sealing by the ends of the electrodes and a peripheral matrix between the ends of the electrodes and separators being impregnated with electrolyte to provide the wet seal. This wet seal method is not suitable for high temperature cells using highly corrosive electrolytes. Another wet seal suitable for low temperature fuel cells is taught by U.S. Pat. No. 4,259,389 to be made of granular inert material bonded with polytetrafluoroethylene, which again is not suitable for high temperature fuel cells.